1. Field of the Invention
This invention relates to methods and apparatus for measuring the power spectrum of optical signals, and more particularly to methods and apparatus for measuring the power spectrum of optical signals by coupling a power of at least one wavelength of the optical signal from a first mode to a second mode and measuring the power spectrum of the coupled optical signal.
2. Description of Related Art
In modern telecommunication systems, many operations with digital signals are performed on an optical layer. For example, digital signals are optically amplified, multiplexed and demultiplexed. In long fiber transmission lines, the amplification function is performed by Erbium Doped Fiber Amplifiers (EDFA""s). The amplifier is able to compensate for power loss related to signal absorption, but it is unable to correct the signal distortion caused by linear dispersion, 4-wave mixing, polarization distortion and other propagation effects, and to get rid of noise accumulation along the transmission line. For these reasons, after the cascade of multiple amplifiers the optical signal has to be regenerated every few hundred kilometers. In practice, the regeneration is performed with electronic repeaters using optical-to-electronic conversion. However to decrease system cost and improve its reliability it is desirable to develop a system and a method of regeneration, or signal refreshing, without optical to electronic conversion. An optical repeater that amplifies and reshapes an input pulse without converting the pulse into the electrical domain is disclosed, for example, in the U.S. Pat. No. 4,971,417, Radiation-Hardened Optical Repeaterxe2x80x9d. The repeater comprises an optical gain device and an optical thresholding material producing the output signal when the intensity of the signal exceeds a threshold. The optical thresholding material such as polydiacetylene thereby performs a pulse shaping function. The nonlinear parameters of polydiacetylene are still under investigation, and its ability to function in an optically thresholding device has to be confirmed.
Another function vital to the telecommunication systems currently performed electronically is signal switching. The switching function is next to be performed on the optical level, especially in the Wavelength Division Multiplexing (WDM) systems. There are two types of optical switches currently under consideration. First, there are wavelength insensitive fiber-to-fiber switches. These switches (mechanical, thermo and electro-optical etc.) are dedicated to redirect the traffic from one optical fiber to another, and will be primarily used for network restoration and reconfiguration. For these purposes, the switching time of about 1 msec (typical for most of these switches) is adequate; however the existing switches do not satisfy the requirements for low cost, reliability and low insertion loss. Second, there are wavelength sensitive switches for WDM systems. In dense WDM systems having a small channel separation, the optical switching is seen as a wavelength sensitive procedure. A small fraction of the traffic carried by specific wavelength should be dropped and added at the intermediate communication node, with the rest of the traffic redirected to different fibers without optical to electronic conversion. This functionality promises significant cost saving in the future networks. Existing wavelength sensitive optical switches are usually bulky, power-consuming and introduce significant loss related to fiber-to-chip mode conversion. Mechanical switches interrupt the traffic stream during the switching time. Acousto-optic tunable filters, made in bulk optic or integrated optic forms, (AOTFs) where the WDM channels are split off by coherent interaction of the acoustic and optical fields though fast, less than about 1 microsecond, are polarization and temperature dependent. Furthermore, the best AOTF consumes several watts of RF power, has spectral resolution about 3 nm between the adjacent channels (which is not adequate for current WDM requirements), and introduces over 5 dB loss because of fiber-to-chip mode conversions.
Another wavelength-sensitive optical switch may be implemented with a tunable Fabry Perot filter (TFPF). When the filter is aligned to a specific wavelength, it is transparent to the incoming optical power. Though the filter mirrors are almost 100% reflective no power is reflected back from the filter. With the wavelength changed or the filter detuned (for example, by tilting the back mirror), the filter becomes almost totally reflective. With the optical circulator in front of the filter, the reflected power may be redirected from the incident port. The most advanced TFPF with mirrors built into the fiber and PZT alignment actuators have only 0.8 dB loss. The disadvantage of these filters is a need for active feedback and a reference element for frequency stability.
There is a need for a method and apparatus that measures and detects the power spectrum of an optical signal. There is a further need for a polarization independent spectral monitor. There is yet a further need for a spectral monitor with high amplitude accuracy.
Accordingly, an object of the present invention is to provide a method and apparatus to measure the power spectrum of a coupled optical signal.
Another object of the present invention is to provide a method and apparatus for monitoring a power spectrum of a coupled optical signal which is substantially independent of the optical polarization state.
A further object of the present invention is to provide a method and apparatus for monitoring a power spectrum of a coupled optical signal with high amplitude accuracy.
These and other objects of the present invention are achieved in a method of measuring a power spectrum of an optical signal. The optical signal is transmitted through an optical fiber. A power of at least one wavelength of the optical signal is coupled from a first mode to a second mode of the waveguide. The power of the optical signal coupled from the first mode to the second mode is measured at a detector.
In another embodiment of the present invention, a method of monitoring a power of an optical signal includes changing polarizations of the optical signal in a polarization scrambler. A first mode of the optical signal is coupled to a second mode at a mode converter. The second mode is detected at a detector. A signal is generated that is responsive to detection of the second mode. The signal is averaged over various polarization states to measure a power that is substantially polarization independent.
In another embodiment of the present invention, a spectral monitor includes an optical fiber with multiple modes. A mode coupler is coupled to the optical fiber. The mode coupler is configured to provide at least one perturbation in the optical fiber to create a coherent coupling between a first mode to a second mode in the optical fiber. A detector is positioned to detect power coupled from the first mode to the second mode. A feedback control is coupled to the mode coupler and the detector to control the power of the coupling power.
In another embodiment of the present invention, a spectral monitor includes an optical fiber with multiple modes and a mode coupler coupled to the optical fiber. The mode coupler is configured to provide at least one perturbation in the optical fiber to create a coherent coupling between a first mode to a second mode in the optical fiber. A core blocking member is positioned at the distal end of the optical fiber. The core blocking member is configured to substantially block those portions of the first mode that are not coupled to the second mode.
In another embodiment of the present invention, a polarization independent spectral monitor includes an optical fiber with multiple modes. A first mode coupler is coupled to the optical fiber. The first mode coupler produces a first acoustic wave in the optical fiber to couple a first mode of an optical signal to a second mode in the optical fiber. A second mode coupler is coupled to the optical fiber. The second mode coupler produces a second acoustic wave in the optical fiber that is orthogonal to the first acoustic wave in order to couple the first mode to the second mode.
In another embodiment of the present invention, a polarization independent spectral monitor includes a mode coupler coupled to an optical fiber with multiple modes. The mode coupler is configured to produce independent orthogonal acoustic waves in the optical fiber that couple a first mode to a second mode. A detector is positioned to detect a coupling power of the coupling from the first mode to the second mode.